Parrot is a virtual machine designed to efficiently compile and execute bytecode for dynamic languages. Parrot currently hosts a variety of language implementations in various stages of completion, including Tcl, Javascript, Ruby, Lua, Scheme, PHP, Python, Perl 6, APL, and a .NET bytecode translator. Parrot is not about parrots, though we are rather fond of them for obvious reasons.

I've spent my Parrot time for the last week working on something I call Alarms. Alarms are like Timers, except instead of specifying a delay the user specifies when they want the Alarm to fire. This is a pretty simple concept. If the Scheduler did what I wanted, I'd be done long ago.

After taking some time off GSoC work to pay the parrot Upgrade Tax that plumage had accumulated in the months since 2.3 shipped, and watching the World Cup this hasn't been an awfully productive week, code-wise. However, there's been quite a bit of work and planing done anyway.

Earlier this summer, when I was considering what sort of interface would be best for the PAST pattern matching and transformation I've been building, Bacek suggested an XPath-like syntax. XPath is, nominally, a language for pattern matching on XML, but it is applicable to any tree-like data structure where each node may have both attributes and children. It's a general-purpose tree-matching language.

In my work on Timers, I've concluded that the scheduler really is the right place for scheduling things (who would have guessed?). How the scheduler *should* work is described in pdd25. How the scheduler actually works is a bit different, and how I want the scheduler to work to implement green threads is a bit different from that...

In my previous post, I described in tutorial form how to implement a very simple constant folding optimization for Integer addition. In this very brief post, I describe in greater detail how that optimization works.

The next step in green threads is to make them preemptive: after one thread has run for a while, it needs to be stopped so a waiting thread can have a turn. Parrot has a mechanism for doing things after a set time called Timers which would be perfect for this. Unfortunately, they don't really work.

Since my last post I haven't spent as much time as I'd like coding, since getting a win32 development environment setup was a bigger time sink than I expected, but I've still managed to stay pretty close to the schedule. The main feature for the next week, once I'm done with the charset-level stuff will be Iterators, which will finally enable NFG 'literals' in PIR source, for which I have added, failing, tests the past week.

The NCI updates using the libffi are coming along. I ran into a bit of an annoyance when I found out that there are places internally where signatures are not specified for some functions, but luckily it was easy to remedy once I had figured out the problem. All of those cases used the same signature ("vJP"), it was only hard to find the right place to add that.

Parrot Abstract Syntax Trees(PASTs) are one of the forms code being compiled in a PCT-based compiler takes before being evaluated or compiled to bytecode. With PAST::Pattern, you can find all the sub-trees of a PAST that match a certain pattern and transform them. This can be used to perform optimizations at the PAST level.